Wavelength division multiplexed (WDM) optical communication systems are known in which multiple optical signals, each having a different wavelength, are combined onto a single optical fiber. Such systems typically include a laser associated with each wavelength, a modulator configured to modulate the output of the laser, and an optical combiner to combine each of the modulated outputs. Conventionally, WDM systems have been constructed from discrete components. For example, the lasers, modulators and combiners have been packaged separately and mounted on a printed circuit board. More recently, however, many WDM components including transmitters, receivers and passive devices have been integrated onto a single chip also referred to as a photonic integrated circuit (PIC).
Differential Quadrature Phase-Shift Keying (DQPSK) has been proposed as an optical signal modulation format capable of increasing the capacity of fiber optic communication systems. Demodulation of DQPSK signals or other signals transmitted in accordance with a self-coherent scheme require a considerable amount of optical and electrical circuitry. Conventional receiver implementations require several PLCs coupled together. Each PLC would include only one component of the optical receiver due to the size of the components. However, implementing an optical receiver requires tight tolerances to properly align the PLCs to avoid signal loss as the optical signals travel from one PLC to another. Additionally, implementing an optical receiver on several PLCs requires a large footprint for the optical circuit which not only requires the overall system to be larger, but also increases manufacturing costs. Accordingly, an improved optical receiver circuit is desirable.